The invention relates to a method for checking the capacitance of a storage capacitor provided in an occupant protection system. Furthermore, the invention relates to a test apparatus for checking the capacitance. The invention is concerned, in particular, with occupant protection systems in motor vehicles, such as restraint systems, for example seatbelt pretensioner systems, or airbag systems.
An occupant protection system of a motor vehicle must trigger reliably in the event of an accident. To that end, one or a plurality of storage capacitors is or are provided, and it or they ensure sufficient power for the firing of firing pellets, by way of example. The power supply is also ensured in an emergency in the event of the failure of an energy source operated from the on-board electrical system. Such storage capacitors can also be used as auxiliary voltage sources for electronic circuits or the like. In order to guarantee the functionality of the occupant protection system, it must be ensured that the capacitance of the storage capacitor used, in particular, the firing capacitor is sufficient. Therefore, it is necessary to check the capacitance of such storage capacitors after the latter have been fitted or else after an occasion or each occasion where the occupant protection system has been activated.
Published, Non-Prosecuted German Patent Application DE 195 08 850 A1 discloses a method for checking the capacitance of a storage capacitor and a test apparatus. Accordingly, the capacitance of the storage capacitor is checked by the storage capacitor, which is charged by a voltage source after activation, and then being disconnected from the voltage source and discharged by a discharge circuit with a defined discharge current over a specific test time interval or measurement duration. From the discharge current and the test time, the change in the capacitor charge is determined by multiplication and divided by an acquired change in the capacitor voltage, in order to determine the capacitance in this way. In the event of an erroneous determination of the capacitance, there is the risk of the storage capacitor being discharged to such an extent that it is no longer functional. Therefore, the known test apparatus contains a protection circuit that terminates the determination of the capacitance if the capacitor voltage falls below a minimum value. Since the storage capacitor must still be able to supply sufficient energy at the minimum voltage value as well and must be able to be charged to a higher voltage value, from which it is discharged in order to measure the capacitance, with the available voltage source, it has to be dimensioned to be larger than is actually necessary, which increases the space requirement of the known test apparatus. The additionally required circuits, that is to say the protection circuit and the discharge circuit, necessitate further outlay and a greater space requirement of the test apparatus. Moreover, the test operation requires a relatively long time owing to the charging, discharging and renewed charging of the storage capacitor to the operating-voltage. Overall, the known method is complicated and the test device that can be used for that purpose is very complex and therefore expensive. The most serious disadvantage of the known capacitance checking, however, is the consumption of valuable stored energy necessitated by the measurement.
It is accordingly an object of the invention to provide a method for checking the capacitance of a storage capacitor provided in an occupant protection system, and also a test apparatus which overcomes the above-mentioned disadvantages of the prior art methods and devices of this general type, which method and which apparatus permit checking without the consumption of the energy stored in the storage capacitor after charging, and are associated with a low outlay.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for checking a capacitance of a storage capacitor including a firing capacitor for firing a firing pellet of an occupant protection system, which includes:
determining a quotient from a change in a charge of a storage capacitor and an associated change in a voltage of the storage capacitor during a charging phase of the storage capacitor; and
determining a capacitance of the storage capacitor from the quotient and the associated change in the voltage of the storage capacitor.
According to the invention, a determination of the capacitance is implemented on a rising edge of the capacitor voltage curve in the charging phase. As a result, the measured storage capacitor is fully functionally available after the determination of the capacitance, without having to be recharged. The charge change in the capacitor charge which is necessary for determining the capacitance is in this case determined by way of the charging current rather than by way of a defined discharge current. The charging current, which flows in any case during a charging phase from a charge carrier source to the capacitor, does not have to be generated in addition, but rather is merely determined during the measurement duration in order to find the change in the capacitor charge. If the change in the capacitor voltage that has occurred during the measurement duration is additionally determined, the capacitance of the capacitor can be calculated from the quotient of the changes of capacitor charge and capacitor voltage.
Unlike in previously known methods, an undesirably non-timely ending of the measurement or determination of the capacitance has no influence on the functionality of the storage capacitor, since the latter, with or without measurement, is charged completely. The invention enables the capacitance to be determined without a separate discharge circuit and protection circuit. The storage capacitor need not be overdimensioned for reasons of measuring the capacitance. Overall, a test apparatus with little additional space requirement and outlay on hardware can thus be realized.
In a preferred embodiment, the charging current is limited by a defined charging resistor between the charge carrier source and the storage capacitor, with the result that the charging phase is extended and sufficient time and more favorable current and/or voltage profiles are available for the determination of the capacitance. The charging current is then preferably determined by way of a measured voltage drop across the charging resistor. The charge change in the capacitor charge is preferably obtained by at least approximate integration of the charging current over the measurement duration, i.e., in the case of a voltage tap on the charging resistor, by integration of the voltage drop over the test time interval and division by the resistance of the charging resistor.
By the determination of the capacitor voltage in the charging phase, which determination is effected in order to ascertain the voltage change in the capacitor voltage, it is also additionally possible, in a preferred refinement of the invention, to monitor the charging operation and, for example, to ascertain early on whether the charging path is interrupted or whether or when the storage capacitor has stored sufficient energy.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for checking the capacitance of a storage capacitor provided in an occupant protection system, and also a test apparatus, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.